HOUSTON, Texas (CNN) -- The National Weather Service in Houston told residents along Galveston Bay on Thursday night they "face certain death" if they don't leave home before Hurricane Ike roars ashore.

Rarely do forecasters use such forceful language.

In fact, the last time they did was three years ago as Hurricane Katrina closed in on New Orleans and the Gulf coast.

Forecasters expect the powerful Category 2 storm to strengthen before its center makes landfall late Friday or early Saturday. The storm is so big that it fills most of the Gulf of Mexico.

Roughly 3.5 million people live in the storm's impact zone, according to federal estimates.

The weather service painted a vivid picture in its warning of the destruction it expects: a towering wall of water -- possibly up to 22 feet high -- crashing over the Galveston Bay shoreline as the brunt of Ike comes ashore. That wall of water could send floodwaters surging into Houston, more than 20 miles inland.

"All neighborhoods ... and possibly entire coastal communities ... will be inundated during the peak storm tide," the weather service warned. "Persons not heeding evacuation orders in single family one or two story homes will face certain death."

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The August 2005 warning for Katrina said "most of the area will be uninhabitable for weeks ... perhaps longer" and that people and animals "exposed to the winds will face certain death."

Shocking at the time, the warning proved largely correct. Parts of New Orleans and the Gulf coast still bear the scars of Katrina and remain uninhabitable.

More than 1,800 people died after Hurricane Katrina. Hundreds more were never accounted for.

What makes Hurricane Ike so intimidating to forecasters involves the location they expect it to make landfall -- near Galveston Island, just south of Houston.

If that happens -- hurricane tracks are hard to predict and subject to change -- the storm's counter-clockwise rotation would push water into Galveston Bay for hour upon hour, battering sea walls and structures.

The final storm surge -- the one that could exceed 20 feet in height -- would come as the eye of crosses the shoreline.

A slight northward change in Ike's path could spare much the Houston area and its millions of residents from catastrophic flooding by keeping the surge out of the bay and pushing it to less-populated areas.